Pulmonary Function Tests (PFTs) are a group of non-invasive breathing tests that provide an objective assessment of how well the lungs are working. These tests measure lung volume, capacity, rates of flow, and gas exchange. A healthcare provider typically orders a PFT to investigate symptoms like a chronic cough or unexplained shortness of breath. The results help diagnose conditions such as asthma, Chronic Obstructive Pulmonary Disease (COPD), or pulmonary fibrosis, and are sometimes performed before a major surgery to assess risk.
Establishing the Baseline for Interpretation
Interpreting a PFT report begins not with your actual measurement, but with understanding the predicted value for a healthy person like you. This baseline, known as the “Predicted Value,” is a statistical average derived from large populations of healthy non-smokers. These reference equations take into account specific biological factors, including your age, sex, height, and ethnicity, as these variables naturally influence lung capacity.
The most reported figure is the “Percent of Predicted,” which is calculated by dividing your measured result by the Predicted Value. A value of 100% means your lung function matches the average for a person with your characteristics, while a result below 80% is conventionally flagged as abnormal. Modern interpretation often uses the Lower Limit of Normal, or Z-score, which represents the fifth percentile and offers a more precise, age-appropriate cutoff, but the 80% threshold remains a common reference point.
Decoding the Primary Spirometry Values
The first primary metric is the Forced Vital Capacity (FVC), representing the total volume of air you can forcefully exhale after taking the deepest possible breath. This measurement reflects the overall size and capacity of the lungs.
The second critical value is the Forced Expiratory Volume in 1 Second (FEV1), which quantifies the amount of air you can push out during that first second of the forced exhalation. The FEV1 is particularly sensitive to changes in the airways and indicates how quickly air can be expelled. A low FEV1 suggests a problem with airflow, such as airway narrowing.
The relationship between these two values is synthesized in the FEV1/FVC Ratio, which is the percentage of the total air you exhaled that came out in the first second. This ratio is the single most informative measurement for determining the pattern of potential lung dysfunction. A healthy adult typically has an FEV1/FVC ratio between 70% and 80%, meaning most of the air capacity is expelled almost immediately.
Identifying the Patterns of Lung Disease
An Obstructive Pattern is defined by a low FEV1/FVC ratio, typically below 70%. This pattern means the patient struggles to get air out quickly, suggesting conditions like asthma or COPD where the airways are narrowed or blocked. In this case, the FEV1 is reduced disproportionately more than the FVC.
Conversely, a Restrictive Pattern is suggested when the FVC is low but the FEV1/FVC ratio is normal or even high. Here, the lungs cannot fully expand, perhaps due to stiffness in the lung tissue itself or a problem with the chest wall. The patient can exhale the air quickly, but the total volume of air they can inhale and exhale is reduced.
A Mixed Pattern is diagnosed when both the FEV1/FVC ratio and the FVC are reduced, indicating both airflow obstruction and reduced lung volume. Once a pattern is identified, the severity is commonly classified based on the degree of reduction in either the FEV1 for obstructive disease or the FVC for suspected restrictive disease. For example, an FEV1 between 60% and 69% of the predicted value is generally categorized as moderate obstruction.
Beyond Spirometry: Lung Volumes and Gas Exchange
The Total Lung Capacity (TLC) test measures the total amount of air the lungs can hold when fully inflated. A low TLC is necessary to confirm a true Restrictive Pattern, as a low FVC alone could sometimes be due to a poor effort during the spirometry maneuver.
The Diffusion Capacity of the Lung for Carbon Monoxide (DLCO) is a separate test that measures how efficiently oxygen moves from the air sacs into the bloodstream. A low DLCO indicates damage to the alveolar-capillary membrane, which is the gas exchange surface of the lung. This finding is often associated with conditions like emphysema or interstitial lung disease.
If an obstructive pattern is found, a Reversibility Test is performed by administering a bronchodilator medication and repeating the spirometry. A positive response, typically defined as a significant increase in FEV1 or FVC, indicates that the airway obstruction is at least partially reversible. This reversibility is a characteristic finding often seen in asthma.